I ain't an EE, either, but I'm pretty sure the answer boils down to "that's just how transistors work". So you have: - Some external power source into the MOS

I ain't an EE, either, but I'm pretty sure the answer boils down to "that's just how transistors work". So you have: - Some external power source into the MOSFET's source pin - VBUS into the MOSFET's gate pin - The MOSFET's drain pin into VSYS And by some transistor magic that I as a lowly layman ain't privy to, the MOSFET only allows power to flow into the input/gate pins and out of the output pin, and prevents power from flowing back out of the input/gate pins. The datasheet does specify a P-channel MOSFET, so that might have something to do with it (the differences between P-channel and N-channel MOSFETs are a bit over my head). If you wanted extra assurance that power won't flow back into VBUS, you could probably stick a diode between VBUS and the MOSFET's gate pin.